In the theory of polycyclic conjugated molecules, several remarkable results are known relating the properties of a phenylene with the analogous properties of a benzenoid molecule that in a natural way is associated with PH, called the hexagonal squeeze (HS) (for details see Fig. 1 or ref. 2). In the present work, the relationships between the energy, E, and the Estrada index, EE, of phenylenes and their hexagonal squeezes are examined. Within sets of isomers, a good linear correlation exists between E(phenylene) and E(hexagonal squeeze), as well as between EE(phenylene) and EE(hexagonal squeeze). The details of these correlations are established. Results show that an earlier obtained relationship between E(phenylene) and E(hexagonal squeeze) needs to be modified.

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